PROVIDED. ADPKD and ARPKD) are important causes of ESRD. In the past cycle, we described a PKD model caused a mutation in Pkhdl (ortholog of the gene mutated in ARPKD), reported that vascular smooth muscle cell from Pkd2+/~ mice have reduced intracellular Ca2+ ([Ca2+]j), sarcoplasmic reticulum Ca2+ stores and capacitative Ca2"1" entry, and provided evidence for activation of cAMP and Ras/MAPK/ERK signaling anc relative inhibition of Akt signaling in PKD. We found that most cysts in PCK rats and Pkd2'/WS25 mice derive from collecting ducts and that administration of antagonists (OPC-31260 and OPC-41061) of the vasopressin V2 receptor (the main adenylyl cyclase agonist in the principal cells) lowers cAMP levels and inhibits PKD development and progression. Hypothesis 1: Alterations in [Ca2+]j homeostasis caused by polycystin or fibrocystin mutations disrupt the functional coordination of cAMP and Ras signaling, hinder negative feedback mechanisms that normally control their activation, and result in a cystic phenotype characterized by abnormal fluid secretion/reabsorption and increased rates of cell proliferation and apoptosis. To address this hypothesis we will determine a) whether reductions in PCI, PC2 or FC expression reduce [Ca2+]j anc mdoplasmic reticulum stores and inhibit capacitative Ca2+ entry in principal cells;b) what mechanismsare responsible for the increased cAMP levels in PKD, c) what mechanismsare responsible for the activation of Ras/MAPK signaling in PKD;d) whether andhow alterations in Ca2+, cAMP or Ras/MAPK signaling disrup the translocation of AQP2 into the apical membrane of principal cells, and e) what are the mechanism responsible for activation of Rho signaling in PKD. Hypothesis 2: Interventions that correct or enhancethe alterations in [Ca2+]j, cAMP, Ras/MAPK or downstream cascades associated with the cystic phenotype wi nhibit or aggravate PKD development and progression. To address this hypothesis we will a) generate female Pkd2M325M2R+l'mice and PCK Brattleboro rats to determine whether the protective effect of OPC 31260 and 41061 is due to V2R antagonism;b) compare the effects of selective vasopressin V2 or V1j receptor antagonists or both in combinationto determinewhether Via antagonismcontributes positively o negatively to the observed beneficial effects;c) determine whether calcimimetic drugs inhibit PKD development;and progression d) determine whether drugs acting on effectors of cAMP-PKA, Ras, affec KD developmentand progression, and e) determine whether L-type calcium channel blockers or a nhibitors have a detrimental effect on PKD developmentand progression.